If that is Singulair you are asking about, I see no info suggesting that as a side effect. If you are taking thyroid med, I suspect that the effects of hypothyroidism or related low ferritin are the most likely causes for hair loss. If you will please tell us about any other symptoms you have, and also post your thyroid related test results and reference ranges shown on the lab report, we will be better able to assess your situation. Also please tell us about your thyroid med and daily dosage.